Process for producing an electrostatic charge image



PROCESS FOR PRODUCING AN ELECTROSTATIC CHARGE IMAGE Filed DeC. 23, 1963GENE H. ROB/NSOA/ INVENTOR.

Ar TOR/VEYS United States Patent Filed Dec. 23, 1963, Ser. No. 332,695 2Claims. (Cl. 96-1) The present invention relates to xerography andxeroprinting and more particularly to a process and apparatus forcharging xerographic and Xeroprinting plates.

Various systems for electrostatically charging xerographic andxeroprinting plates, hereinafter referred to as xero plates, are known.The most common systems employ fine wires held at corona-producingpotentials. Other systems employ materials of intermediate conductivitybridging the space between an electrode and surface of the plate to becharged. All such systems require means for moving the charger and theplate to be charged relative to each other to achieve uniformity ofcharging. Such relative movement is customarily achieved by sweeping thecharging device over the surface to be charged.

It is the primary object of the present invention to provide a simplemethod for applying in a uniform manner an electrostatic charge to theentire surface of a xero plate without requiring any relative movementbetween the charging device and the xero plate.

It is another object of the invention to provide a simple method ofelectrostatically charging a xerographic plate which is particularlyadapted for use with reflex printing.

It is a still further object of the invention to provide a method ofelectrostatically charging a xerographic plate in which the chargingstep itself effects reflex exposure of the plate.

It is another object of the invention to provide a method and apparatusfor electrostatically charging a xerographic sheet while it is incontact with a document to be reproduced.

These objects are accomplished by the following invention. The processof the invention comprises placing a sheet of paper, or material ofsimilar conductivity, in contact with a xero plate, sandwiching thesheet and the plate between parallel, plate electrodes, and applying apotential difference of about 1500 volts between the electrodes forapproximately one second. In one embodiment the sheet is the document tobe reproduced, and by having the appropriate electrode transparent, areflextype of exposure is made without disassembling the sandwich. In aspecies of this embodiment, prolonged charging itself effectssimultaneous reflex exposure of the xerographic sheet.

These and other objects of the invention will be more fully understoodfrom the following description, when read in connection with theaccompanying drawing, in which:

FIG. 1 is a cross-sectional view through a xerographic plate and adocument sandwiched between a pair of plate electrodes;

FIG. 2 is a cross-sectional view through a xerographic plate and a layerof cellophane sandwiched between a pair of plate electrodes;

FIG. 3 is a diagrammatic view of an apparatus for electrically chargingand reflex exposing successive portions of a xerographic sheet accordingto the present invention; and

FIG. 4 is a cross-sectional view through a xeroprinting plate and asheet of paper sandwiched between a pair of plate electrodes.

FIG. 1 shows a xerographic plate 1 which may consist of a support 3 suchas paper, coated with a zinc-oxide-inresin-binder photoconductive layer5. The plate 1 is 3,2713% Farmed Sept. 6, tees ICC placed in contactwith a document 7, to be reproduced. The xerographic plate 1 'and thedocument 7 are sandwiched between the plate electrodes 9 and 11. Avoltage source 13 and a switch 15 are connected between the electrodes 9and 11. The electrode 9 is connected to the negative terminal of thevoltage source 13 and the posi tive terminal and the electrode 11 areconnected to ground. In addition to charging the xerographic plate 1,the arrangement shown in FIG. 1 is also adapted to permit reflexexposure of the xerographic plate 1. By having the appropriateelectrode, in this case the electrode 11, transparent, the xerographicplate 1 can be reflex exposed, for example by means of a light source17, after or during and after charging, without having to disassemblethe sandwich. The sandwich can then be disassembled for xerographicdevelopment of the electrostatic image on the xerographic plate 1, byany of the known methods, such as cascade, magnetic brush, etc.

FIG. 2 shows a xerographic plate 2% consisting for example of a papersupport 22 having a photoconductive layer 24, such asZinc-oxide-in-resin binder, coated thereon. The photoconductive side ofthe plate 20 is placed in contact with a transparent sheet 26 ofmaterial having the appropriate conductivity. The plate 20 and the sheet26 are sandwiched between an upper plate electrode 28 and a lower plateelectrode 30. A voltage source 32 and a switch 34 are connected acrossthe plate electrodes 28 and 30. The positive terminal of the voltagesource 32 is connected to the upper electrode 28 and to ground and thenegative terminal is connected to the lower electrode 30. By having thelower electrode 30 transparent, the photoconductive layer 24 can beprojection exposed, after charging, without disassembling the sandwich,by means of a lamp 36, a transparency 38, and a lens 40. The sandwichcan then be disassembled and the electrostatic image xerographicallydeveloped.

nected to the electrode 54 and the positive terminal of the voltagesource 58 and the electrode 56 and connected to ground. A document 62 tobe reproduced is sandwiched between the electrodes 54 and 60 with theimage surface in contact with the xerographic sheet 50. By having thelower electrode 56 transparent, the xerographic sheet 50 can be reflexexposed by means of a light source 64. After the charging and exposingsteps, the xerographic sheet 50 is advanced through the sandwhich toposition a new area of the xerographic sheet 56 for charging andexposing. As the sheet 50 is advanced, the previ ously charged andexposed areas of the xerographic sheet 50 are fed through a station 66for subsequent operations. For example, the electrostatic image is firstdeveloped and the toner particles either fixed to the sheet 50 ortransferred to a permanent record medium and fixed thereto. Thesesubsequent operations are well known in the art, and since they form nopart of the present invention they will not be described in detail. Thexerographic sheet 50 can be advanced through the sandwhich as many timesas is necessary to produce the required number of copies of any givendocument 62, without requiring the sandwich to be disassembled. Theupper electrode 54 is made easily removable, for example, by hinging oneside thereof, to

provide for simple replacement of documents to be reproduced. Thearrangement shown in FIG. 3 for continuous operation withoutdisassembling the sandwhich is also be between about 10 and ohms.

3 useful in the embodiment described with reference to FIG. 2.

According to the present invention the auxiliary sheet, such as thesheet 26 of FIG. 2, should have a resistance per unit area lying betweenthat of metal on one hand, and a good insulator on the other hand. Theresistance between the upper and lower surfaces of an area of theauxiliary sheet of one square centimeter should preferably Too lowresistance leads to non-uniform charging and provides the xero platewith no protection against localized or arc breakdown.

Too high resistance leads to insufficient charging or a very I low rateof charging.

A large variety of paper grades and thicknesses have been used; allworked well. Plastic films of appropriate conductivity, for example,cellophane, gelatin, or glassine, can be used, especially whentransparency is required in the auxiliary sheet. Fabrics such as cotton,wool, linen, and viscose rayon have been successfully employed. Varioustypes of exposure, e.g., reflex, projection, and contact, have beensuccessfully employed in combination with the charging method of thepresent invention.

Two unexpected results arose from the practice of the invention withxerographic sheets having zinc-oxide-inresin-binder photoconductivelayers. First, it was found that uniform sandwhich charging could beaccomplished only when the pigment-to-binder ratio is 3:1 or lower. Suchphotoconductive layers having a higher pigment content would not chargeuniformly and they displayed a mottled appearance when developed.Second, it was found that if charging times were extended toapproximately 30 seconds under the conditions described below in Example1 for reflex exposing, the discharge occurring between thephotoconductive layer and the document not only serves to charge thephotoconductive layer but also serves as a light source supplying enoughenergy in the visible and ultraviolet region of the spectrum to causereflex exposure of the photoconductive layer. A possible explanation forthis phenomenon is that the long exposure required induces in thephotoconductive layer a state of fatigue; 21 state in which the layer isless able to store charge.

Another unexpected result was associated with the use of printeddocuments as the auxiliary or buffer sheets. It was expected that theprinted matter would interfere with uniform charging of thephotoconductive layer. However, over a wide range of charging times andelectrode potentials no such interference was found.

FIG. 4 shows a xeroprinting plate 70, which may consist, for example, ofa metal plate 71 having a pattern 72 of insulating resin. The plate 70is placed in contact with a sheet 73 0f paper or other material ofsimilar conductivity. The plate 70 and the sheet 73 are sandwiched,between plate electrodes 74 and 75. A voltage source 76 and a switch 77are connected between the electrodes 74 and 75.

The present invention is further illustrated by the following examplesof preferred embodiments thereof although it will be understood thatthese examples are included merely for purposes of illustration and arenot intended to limit the scope of the invention.

Example N0. 1

A zinc-oxide-in-resin-binder xerographic layer with a pigment-to-binderratio of 3:1 and an original document were placed between a pair ofplate electrodes (one of trodes.

A zinc-oxide-in-resin-binder xerographic sheet with a pigment-to-binderratio of 3:1 was placed between a pair of plate electrodes, one of whichwas transparent and had between itself and the xerographic layer a layerof cellophane, see FIG. 2. Cellophane has a resistivity of the sameorder of magnitude as that of paper. A potential difference of 2500volts was applied to the elec- With the sandwich still assembled, thexerographic sheet was exposed by projection through the transparentelectrode and the cellophane. The sandwich was then diassembled and theelectrostatic image on the xerographic sheet was developed with anelectroscopic powder which was then fused thereon to produce a permanentcopy.

Example N0. 3

A zinc-oxide-in-resin-binder xerographic sheet was placed between a pairof plate electrodes as in Example No. 1, with a piece of paper used inplace of the document. A potential difference of 1500 volts was appliedacross the electrodes as in Example No. 1 and the sandwich was thendisassembled for contact exposure. The electrostatic image on thexerographic sheet was then developed with an electrostatic powder andthe powder was then fused thereon to produce a permanent copy.

Example N0. 4

Kodak Photo Resist was used to produce a pattern of insulating resin ona metal plate. The metal plate was then place-d between a pair ofelectrodes as shown in FIG. 4 with a sheet of paper. A potentialdifference of 1500 volts was applied across the electrodes. The sandwichwas disassembled and the charged resist pattern was developed with anelectroscopic powder. The powder was then electrostatically transferredto bond paper and fused thereto to produce a permanent copy.

Example N0. 6

A dispersion of photoconductive zinc cadmium sulfide (No. 2225, The NewJersey Zinc Company) in a binder (Plaskon ST-856, a silicone-alkydresin, Barrett Division, Allied Chemical Corporation) Was coated on Nesaglass and placed between a pair of electrodes as in Example No. 1 with apiece of paper used in place of the document. A potential difference of1500 volts was applied across the electrodes and the coated Nesa glasswas subsequently processed as in Example No. 3 to produce a permanentprint.

Although the invention has been described in detail with reference tocertain preferred embodiments thereof, it will be understood thatvariations and modifications can be effected without departing from thespirit and scope of the invention as described hereinabove and as'defined in the appended claims.

I claim:

1. A process for producing an electrostatic charge image, correspondingto an image on a document to be recorded, on a xerographic sheetcomprising a photoconductive insulating layer on a conductive backing,comprising the steps of:

(a) positioning said document in facing, virtual contact with saidsheet, said document and said sheet being separated by a minute air gap,with said image of said document in contact with said layer,

(b) sandwiching said document and said sheet between and in contact withtwo plate electrodes,

(c) applying a voltage across said electrodes of a value sufiicient toionize the air in said air gap whereby ions of one polarity deposit onand therefore charge said layer, and

(d) continuing said voltage application for a time sufficient to allowthe light produced by said ionization to reflex expose said layer,whereby an electrostatic charge image, corresponding to said image to berecorded on said document, is produced on said layer without thenecessity for a separate, additional exposure step from an externalsource of actin-ic radiation.

2. The process according to claim 1 in which said document comprises amaterial of intermediate conductivity having a resistance of betweenabout 10 and 10 ohm/cmfl, in which said photoconductive insulating 6layer comprises zinc oxide in resin binder with a pigment to binderratio no greater than 3:1, in which said applying step comprisesapplying about 1500 volts across said electrodes, and in which saidcontinuing step comprises continuing said voltage application for about30 seconds.

References Cited by the Examiner NOR-MAN G. TORC HIN, Primary Examiner.

20 C. E. VAN H'ORN, Assistant Examiner.

1. A PROCESS FOR PRODUCING AN ELECTROSTATIC CHARGE IMAGE, CORRESPONDINGTO AN IMAGE ON A DOCUMENT TO BE RECORDED, ON A XEROGRAPHIC SHEETCOMPRISING A PHOTOCONDUCTIVE INSULATING LAYER ON A CONDUCTIVE BACKING,COMPRISING THE STEPS OF: (A) POSITIONING SAID DOCUMENT IN FACING,VIRTUAL CONTACT WITH SAID SHEET, SAID DOCUMENT AND SAID SHEET BEINGSEPARATED BY A MINUTE AIR GAP, WITH SAID IMAGE OF SAID DOCUMENT INCONTACT WITH SAID LAYER, (B) SANDWICHING SAID DOCUMENT AND SAID SHEETBETWEEN AND IN CONTACT WITH TWO PLATE ELECTRODES,